This application is based on Application No.2001-353130, filed in Japan on Nov. 19, 2001, the contents of which are hereby incorporated by reference.
The present invention relates to a vehicle steering system for steering steerable wheels of a vehicle, and more particularly to a vehicle steering system provided with a sub steering mechanism that is needed in the case where automatic steering for compensating for a steering amount by a driver is performed or steering for automatic driving is performed.
In a vehicle steering system, it has conventionally been proposed to automatically assist and control a direction of steerable vehicle wheels so as to optimize, for example, the steering characteristics (actual change in the vehicle advancing direction with respect to an rotating amount of a steering wheel) or to automatically drive and control a steering system for the driver in order to keep the travel position of the vehicle within a vehicle lane.
As the vehicle steering system for performing such steering control, a number of vehicle steering apparatuses which adopt a steering system provided with a sub steering mechanism that may be electrically controlled, such as that disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 6-206553 or a vehicle steering system called a steering-by-wire in which a mechanical link between the steering wheel and the steered vehicle wheels are eliminated are disclosed.
The above-described conventional vehicle steering system provided with a sub steering mechanism is composed of a structure for adding steering assist by the sub steering mechanism composed of a planetary gear mechanism and the like to the main steering being performed by the driver to thereby steer the steerable vehicle wheels, i.e., a mechanical angle addition mechanism for adding an assist steering angle to the main steering angle.
However, the assist steering torque to be applied by the sub steering mechanism by means of such an angle addition mechanism is distributed by being divided into a steering wheel reactive torque and a torque for steering the steerable vehicle wheels by the sub steering mechanism. Accordingly, the steering wheel reactive force that was not expected by the driver is generated when the sub steering mechanism is driven.
The driver performs, in response to that steering wheel reactive force, various steering operations which can not be uniquely determined, such as xe2x80x9ccontinuing steering of the steering wheel regardless of the change of the steering wheel reactive forcexe2x80x9d, xe2x80x9csteering the steering in a manner such as to counter the change of the steering wheel reactive forcexe2x80x9d, xe2x80x9csteering the steering wheel in a direction in which the reactive force is generated, in response to the change of the steering wheel reactive forcexe2x80x9d.
For example, in the case where a driver intends to achieve greater steering than that attained by steering of the steering wheel by means of the sub steering mechanism while increasing the steering amount of the steering wheel, the driver feels that the steering wheel reactive force becomes heavier as he continues to increase the steering of the steering wheel. In contrast, when the driver returns the steering wheel back, the steering angle of the steerable vehicle wheels, which is expected to be obtained by adding the assist steering angle by the sub steering mechanism, becomes insufficient by an amount equivalent to the amount of steering which the driver has returned by steering the steering wheel back.
Accordingly, under the condition that the steering wheel operation by the driver performed in response to the steering wheel reactive force generated by driving of the sub steering mechanism is not uniquely determined, it is difficult to automatically control the direction of the steerable vehicle wheels for the above-described reason. Also, in the case where the above-described steering wheel reactive force is compensated for by another actuator as described in Japanese Patent Application Laid-Open No. Hei 6-206553, it is difficult to completely eliminate the steering wheel reactive force due to control lag or the like. Therefore, this is not a perfect countermeasure for the foregoing problem.
On the other hand, in the steer-by-wire system, the steering wheel reactive force to be transmitted to the driver through the steering wheel is determined according to the condition of the vehicle and is given to the steering wheel by the reactive force system. Also, the steering wheel operation by the driver is detected. The steering wheel target value is calculated on the basis of the detected steering wheel operation to thereby perform, by the steering system, steering by the actual steering angle. Accordingly, the above-described problem may be overcome.
However, provision of a mechanical stand-by back-up mechanism (for example, connection between the steering wheel shaft and the actual steering shaft through a clutch) for transmitting the driver""s steering wheel operation to the steerable vehicle wheels when the apparatus is broken down is essential. Even if the back-up system is assembled in, there is a disadvantage in that the steering operation is disabled for a brief moment upon gear switching until the connection is restored by means of the clutch.
Also, in the steer-by-wire system, there is no mechanical link between the steering wheel and the steerable vehicle wheels. Therefore, there is no guarantee that the relationship between the direction of the steering wheel and the direction of the steerable vehicle wheels upon activation of the system is correct. As a result, there is a problem that it is necessary to take a synchronism between the steering wheel angle and the actual steering angle so as to ensure the correct relationship.
Furthermore, in the steering mechanism such as described in Japanese Patent Application Laid-Open No. Hei 6-206553, since the mechanical link mechanism exists from the steering wheel to the steered vehicle wheels, there is a risk that the steering wheel would harm the driver due to rearward movement of the steering mechanism upon collision of the vehicle.
In order to overcome the above-described defects, an object of the present invention is to provide a steering system that may simultaneously allow manual steering and automatic steering, and in addition ensure the same steering feeling as normal even if the manual steering and automatic steering are simultaneously performed.
According to the present invention, there is provided a vehicle steering system including a steering mechanism for steering steerable vehicle wheels in accordance with a rotating amount of a steering wheel and a biasing amount given by a sub steering mechanism for compensating for the rotational amount of the steering wheel by applying a drive torque thereto. The system comprises a target reactive force generating means for generating a target value of a reactive torque to be applied to the steering wheel. Also provided are a reactive torque detecting means for detecting the reactive torque being applied to the steering wheel; a reactive torque controlling means for controlling the drive torque to be applied to the sub steering mechanism so that a target torque generated by the target reactive force generating means and the reactive torque detected by the reactive torque detecting means become equal to each other. The system comprises a steerable vehicle wheel controlling mechanism for controlling a steering direction of the steerable vehicle wheels; an actual steering angle detecting means for detecting an actual steering angle of the steerable vehicle wheels to be controlled by the steerable vehicle wheel controlling mechanism. Also provided are a target steering angle generating means for generating a target value of the steering angle of the steerable vehicle wheels; and an actual steering angle controlling means for driving and controlling the steerable vehicle wheel controlling mechanism so that the target steering angle generated by the target steering angle generating means and the actual steering angle detected by the actual steering angle detecting means becomes equal to each other.
In the vehicle steering system in accordance with the present invention, it is possible to simultaneously execute the automatic steering operation on the basis of the target steering angle generating means with the manual steering operation by the steering wheel and to provide the steering feeling as normal. Therefore, it is possible to simultaneously execute the automatic steering operation on the basis of the target steering angle generating means with the manual steering operation by the steering wheel and to provide the steering feeling as normal.